1. Field of the Invention
The present invention is a continuation in part of my U.S. patent application Ser. Nos. 07/977,138 filed on Nov. 16, 1992, now U.S. Pat. No. 5,321,946, and 08/011,870 filed on Feb. 1, 1993, now U.S. Pat. No. 5,4.03,569 in which I disclosed methods for clean-up of a boiler flue gas stream utilizing cooling and condensing processes to remove its acidic water vapor, and separating sulfur dioxide and carbon dioxide contained therein by thermodynamic equilibria and liquefaction processes. More particularly it relates to a method for removing the sulfur dioxide from the flue gases in a centrifugal cyclonic gas separator-absorber by a regenerative organic amine absorbent that can be recovered, purified and recycled. The absorbed sulfur dioxide is stripped from the absorbent, dehumidified, liquefied and recycled.
2. Description of the Prior Art
The present invention employs a thermally regenerative organic based amine absorbent to absorb most of the sulfur dioxide contained in the flue gases in a cyclonic centrifugal gas separator-absorber. The SO.sub.2 contained in the flue gas is enriched in a heavier fraction of gas along the perimeter wall of the cyclone structure. The process will enhance absorption and removal of SO.sub.2 at much lower energy consumption rate when compared with other flue gas desulfurization systems. In the past regenerative adsorbents have been used for scrubbing sulfur oxides (SO.sub.x) and nitrogen oxides (NO.sub.x) from flue gases containing carbon dioxide, and other flue gas pollutants. U.S. Pat. No. 3,904,735 disclosed a system for selectively removing sulfur dioxide from a flue gas mixture by employing an alkaline nitrogen based, thermally regeneratable organic amine as Triethanolamine or (CANSOLV.TM.), for an absorbent in a sulfur dioxide recovery scrubbing process. Aqueous scrubbing regenerative cycles have been the basis of most commercial regeneratable flue gas desulfurization (FGD) processes, such as when using sodium sulfite in aqueous media. Sulfur dioxide undergoes reversible hydration and ionization reactions, the sodium ion does not participate in the reaction, its role being to provide electrical neutrality to the solution. The absorption of SO.sub.2 directly with aqueous sodium based liquor requires a large amount of water, which will have to be stripped with the SO.sub.2. Because of the vapor pressure of water in the scrubbing liquor is much higher, co-stripping of a large amount of water wastefully consumes a great amount of energy, this makes many traditional regenerative processes not cost effective.
In the prior art other soluble cation as the triethanolamine ion has been used in enumerated nitrogen based regeneratable absorbent compounds for flue gas desulfurization systems. A commercially known regenerative organic amine absorbent (CANSOLV.TM.) has been developed by Union Carbide Chemicals and Plastics Company. In the CANSOLV.TM. process, the untreated flue gas leaving a particulate removal device; a dust collector or electrostatic precipitator; at above 270.degree. F. is first scrubbed by water, and then by aqueous solution of the SO.sub.2 absorbent, both the water and the SO.sub.2 absorbent are atomized through high pressure nozzles in flue gas duct section, each duct section has a mist elimination equipment to remove the rich SO.sub.2 absorbent liquor droplets from the gas stream. The treated gas is released to the chimney. In the water scrubbing duct section part, the water mist evaporates to effect cooling and humidifying the flue gases. The water absorbs any chlorides, fluorides, sulfurous acid and removes any trace metals and ultra fine particulate matter released from the dust collector. In the SO.sub.2 absorber duct section, the SO.sub.2 is scrubbed from the flue gas by contact with the atomized absorbent mist, the rich SO.sub.2 absorbent liquor is removed and regenerated through application of heat, and stripping of the SO.sub.2. Unlike the present invention, in the CANSOLV.TM. process; large amount of water is needed for scrubbing, cooling, and cleaning the flue gas stream leaving the electrostatic precipitator or any other particulate control device. The present invention provides means for condensing the acidic water in the flue gases through energy recovery, and provides an increased rate of SO.sub.2 absorption by employing a centrifugal gas separator-absorber which increases the gas pressure, and provides cooling for a heavier fraction of gas which contains higher concentration of SO.sub.2.
The present invention is based on employing alkaline organic amine (Triethanolamine or other nitrogen based ion similar to the CANSOLV.TM.) to remove up to 99% of the sulfur dioxide, at a relatively higher flue gas velocity, a relatively lower liquid to gas ratio (L/G), much lower energy consumption rates, and reduced power requirements. When compared with separating and liquefying the sulfur dioxide by processes as disclosed in my U.S. patent application Ser. No. 977,138, the improvement of the present invention is using an organic amine absorbent sprayed inside a cyclonic centrifugal gas separator-absorber. The centrifugal gas separator-absorber features air atomizing nozzles mounted at the perimeter of the upper cylindrical intake chamber of the separator. The increased concentration of the sulfur dioxide in a heavier fraction of gas mixture, the higher pressure and cooling effect of the heavier gas fraction at the perimeter wall of the cyclonic separator-absorber, enhances the absorption of the sulfur dioxide. A heavier gas fraction laden with a rich SO.sub.2 solvent liquor flowing from the bottom of the gas separator-absorber is removed, the rich SO.sub.2 solvent liquor is separated, clarified or filtered, and is then regenerated by heating and stripping into a vacuum stripping column. The stripped SO.sub.2 gas laden with water vapor is removed from the top of the stripping column, and the regenerated absorbent liquor is removed from the bottom and recycled.
Similar to other regenerative organic absorbent processes used in the past, the organic absorbent must be nonvolatile, having a very low vapor pressure to prevent vapor losses in the vacuum stripping step. The absorbent liquor capacity to remove the SO.sub.2, normally decreases in the presence of acids introduced in the absorbent, these acids include sulfuric acid (H.sub.2 SO.sub.4), hydrochloric acid (HCL), fluoride acid (HF), and nitric acid (HNO.sub.3). Unlike the prior art; in the present invention most of these acids are removed from the flue gas stream in a condensing heat exchanger prior to entering the cyclonic gas separator-absorber. This step of the flue gas cooling and condensing, removes most of the acid vapors prior to the absorption step, this minimizes the formation of any heat stable acid salts in the absorber due to reactions of the acids with the amine absorbent.
Organic amine adsorbents can exhibit high selectivity for absorbing sulfur dioxide in the presence of carbon dioxide, oxygen, and other flue gas components. Their low liquid to gas molar ratio (L/G), and their excellent stable stripping characteristics made them particularly appeal to the present invention. To maintain the highest possible absorbing capacity of an amine absorbent, it is important to control the water and acids introduced in the absorbent liquor to a required molar balance with the sulfur dioxide.
Since Organic amine solvents are commercially established and available, the process designer can evaluate and make the selection of the absorbent for the process. The physical and chemical composition of the solvent, its absorption, and regeneration characteristics are considered outside the scope of this application.